This invention relates to the detection of DC ground faults. In particular it relates to an apparatus and a method for detecting such faults in normally ungrounded DC distribution systems having significant capacitive reactance components and strong electromagnetic and electrostatic fields associated with utility power generation and distribution, industrial plants, and computer/electronic systems, where ground faults must be located without taking unaffected equipment out of service.
A basic problem in such systems is the need to identify small DC fault currents namely low to high impedance ground faults in the presence of much larger DC load currents.
One well-known ground detection circuit consists of a center tapped high resistance connected across the DC source and an indicating milliammeter between the center tap and ground. A ground fault anywhere on the DC system causes an indication on the milliammeter. Since the high resistance limits the ground fault current to a few milliamperes the circuit is not tripped off when a fault occurs. This is important since loss of power on a typical DC control circuit is often critical and could involve safety hazards. It is important to locate and repair any ground faults as soon as practical since a second ground fault would trip the circuit.
Such a ground detection system while indicating that a ground fault has occurred does not tell where the fault lies. It could be in any one of many pieces of equipment on numerous branch circuits. Again because of the critical nature of these circuits it is not practical to turn them off one at a time to locate the fault. Thus a system is needed to locate the faulty equipment without interrupting these critical circuits.
Another system for DC fault detection requires the introduction into a DC fault line of an AC current at a frequency of about 25 Hz, which is then detected. A problem with the introduction of such AC current is that it is liable to cause operational problems, mask some faults, and create complications in detecting and localizing ground faults in some DC loads in the system.
It is also known elsewhere to test for DC faults in small systems employing grounded 12-volt battery type power supplies in automobiles and the like. Such grounded DC systems require the connection of an injector across terminals of the battery supply and thereafter a detector is applied over the wiring system with sound detection means such that an increasing sound would indicate where a DC fault exists.
Such systems operate in response to high DC fault currents in an environment where there is no capacitive or inductive reactances of consequence, or where they are of no real concern and where the DC system is effectively shut off when the fault detection is being made.
It is also known in AC systems to detect ground leakage by a relay which interrupts the system so as to introduce a fault current in the sense of a pulsating input. Such systems however are of a nature that a D'Arsonval type meter or permanent magnet moving coil meter are used for detection of the pulsating input. Such a meter requires a current transformer suitable for detecting relatively large AC fault currents, and this is unsuitable for measuring pulsating DC fault currents of a lower value. These detection systems are particularly unsuitable in high electrostatic and electromagnetic environments.